With recent development of scientific technology, a technique for processing a work with ultra-high precision has been increasingly demanded. As electrolytic grinding technique for satisfying such demand, the applicant has developed and suggested electrolytic inprocess dressing (hereinafter, "ELID") grinding ("Latest technology trend on mirror surface grinding", Riken Symposium, Mar. 5, 1991).
In the above mentioned ELID grinding, there is used an electrically conductive grinding wheel instead of an electrode as in conventional electrolytic grinding. In the ELID grinding, an electrode is disposed spaced away from and facing the grinding wheel, and a voltage is applied across the grinding wheel and the electrode while making electrically conductive fluid flow between the grinding wheel and the electrode. A work is ground with the grinding wheel with the grinding wheel being dressed by electrolysis. In ELID grinding, even if a finer abrasive grain is used, blinding does not occur in a grinding wheel due to setting of abrasive grains caused by electrolytic dressing. Hence, the use of finer abrasive grain makes it possible to obtain a superior ground surface like a mirror surface by grinding. Thus, ELID grinding can maintain the sharpness of a grinding wheel in a range from high efficiency grinding to mirror surface grinding, and is expected to be applied to various grinding techniques as a means capable of forming a surface with high accuracy in a short period of time, which was impossible by conventional methods.
Optical elements (such as a lens and a mirror) having aspheric surfaces which are typical of highly precise parts and are required to have not only small surface roughness but also high precision shapes. In order to grind such optical elements with the above mentioned ELID grinding, an electrically conductive grinding wheel formed with a surface having a desired shape (for instance, an aspheric surface) is indispensable. However, a problem has been that such a grinding wheel is difficult to make, takes too much time and is costly. In addition, even if a grinding wheel is formed with a desired surface, the surface shape may be changed due to abrasion and dressing occurring in use, and hence it has been impossible to maintain high accuracy while grinding a work.
Thus, there has been suggested an NC processing apparatus for ELID-grinding a work to thereby have a desired surface shape by numerically controlling a position of a grinding wheel. Such an NC processing apparatus has already been used by some people.
However, there arose a problem that it was impossible to have a desired, high shape accuracy by single grinding due to elastic deformation of a work (material to be ground) and a grinding wheel, even if the above mentioned NC processing apparatus was used. Thus, in order to accomplish high shape accuracy, a dimension of a shape of a ground work was measured, and then NC input data was compensated based on the measurement data. Then, a work was ground repeatedly. Thus, there were problems that it was necessary to repeat grinding a work for obtaining a desired shape accuracy, which took much time, and that compensation failed in many cases since compensation data was established by the intuition of those skilled in the art or by trial and error.
The data obtained by measuring a ground work shape contains various signal elements as well as true signals transmitted from a target. For instance, various signal elements include false signals transmitted from material other than a target, fluctuation in sensitivity of a sensor due to variation of measurement environment, and thermal drift of an electric system. If rough grinding is carried out in an initial stage for obtaining higher efficiency in grinding, the measurement data contains fine signal waves indicating roughness to be obtained when a rough, ground surface is measured. Thus, it is difficult to grasp a true surface shape. For these reasons, there were problems that establishment of compensation data from measurement data was difficult even for those skilled in the art, took much time, and was likely to contain a lot of compensation errors.
In addition, it was necessary in a conventional method for measuring a shape of a ground work to remove the work out of an NC processing apparatus and attach the work to an appropriate measurement device to thereby measure a shape of the work. Then, the work had to be mounted again onto an NC processing apparatus. Since a work may be disposed significantly out of position each time it is mounted/removed, there arose the problem that it was difficult to adjust the position of a work and that such position adjustment took a lot of time.
The present invention is made in order to solve the above mentioned problems. Namely, it is an object of the present invention to provide a method of controlling a shape of a work in ELID-grinding which method is capable of accomplishing higher shape accuracy with a smaller number of processing steps by using an NC processing apparatus, and an NC processing apparatus utilizing the method. Another object of the present invention is to provide a method of controlling a shape of a work which method is capable of extracting true shape signals out of measurement data. A further object of the present invention is to provide an NC processing apparatus for ELID-grinding which is capable of avoiding the mispositioning of a work caused by attaching/removing a work.